X-ray study of baker's yeast lipoamide dehydrogenase at 4.5 A resolution by molecular replacement method.

The molecular structure of lipoamide dehydrogenase from baker's yeast has been determined at 4.5 A resolution by molecular replacement techniques using the known structure of human erythrocyte glutathione reductase as a starting model. The enzyme crystallizes in the space group P2(1)2(1)2(1) with a = 98.6(2), b = 162.0(2), c = 69.4(2) A. There is one molecule per asymmetric unit. The enzyme is a dimeric protein of identical subunits related by a local two-fold symmetry. Comparison of the tertiary structures between glutathione reductase and the present enzyme shows that the folding is almost the same except for the N and C termini, although some slight shortening or shifting of alpha-helices was found in the electron density map. FAD molecules are found at similar positions to those of glutathione reductase. Since the amino acid residues around FAD and NAD binding sites and at the reaction centers of the two enzymes are strongly conserved, the lipoamide dehydrogenase may catalyze the opposite reaction through a similar mechanism to that proposed for glutathione reductase. The newly found C terminus is located near the edge of a deep cave at the interface between the two subunits. These additional 18 residues form a narrow entrance to the cave, in which the long chain of the dihydrolipoyl moiety of lipoate acetyltransferase will be bound.